Spatial Distribution of Molecules in Damped Ly Clouds

　　 　 Hiroyuki Hirashita 　 　 (Nagoya University, Japan / SISSA, Italy)

　 A. Ferrara, K. Wada, P. Richter 　

1. H2 and Dust in DLAs2. Spatial H2 Distribution (Theory)3. Summary

Contents:

1. H2 and Dust in DLAs

Molecular hydrogen (H2)• The most abundant molecule in the

universe: a tracer of cool environments• Molecular clouds are the site of star

formation.

Dust• H2 formation takes place.• Shielding of UV and reprocess into IR

High H I column density (～ 1021 cm-2) High redshift sample with detailed information on ISM.

QSODamped Ly cloudLy absorption

Damped Ly Cloud (DLA)

How about H2 and dust? (e.g., Petitjean et al. 2002)

Correlation :Dust and H2 in DLAs

Ledoux, Petitjean, & Srianand (2003)

log

(mol

ecul

ar f

ract

ion)

metal depletion log (dust/gas)

Correlation between dust abundance and molecular fraction.

H2 is not detected.

Large scatter

◆ Strongly inhomogeneous H2 distribution?

How can we explain it?

UV background UV background

Dust poor Dust rich

Hard to detect H2 rich regions

H2 rich regions

Large change of H2 detection(with large scatter in abundance)

◆Numerical calculation (vcir = 100 km/s, zform = 3)

2. Spatial H2 distributionHirashita et al. (2003)

Density

1 kpc

Temperature

Included physics on H2: (1) Formation on dust grains (2) Dissociation by UV bg (self-shielding included) (1) = (2) i21 = 0.1, D = 0.1 Dsun

Molecular Fraction Map

The H2 distribution is highly inhomogeneous (confined in small clumpy regions).

⇒ Low chance to detect　 H2 from DLAs

H2 distribution (small scale)

H2 rich regions are confined in small regions.The area with fH2 > 10–6 is only 10% of the surface.

50 pc

H2 and Dust

Overall correlationRapid increase of fH2

around log (D/Dsun) ~ –1.5.Large scatter for high D

log (dust-to-gas ratio)

log

(mol

ecul

ar f

ract

ion)

×: Ledoux et al. (2003)◆: our simulation

Random 5 lines of sight through the disc for eachdust-to-gas ratio

(1)The paucity of H2 detection for dust-poor DLAs is explained by the small area covered by H2 rich regions.

(2)The correlation between dust-to-gas ratio and H2 abundance has been explained.

(3)The large variety in H2 fraction for relatively dust-rich DLAs is naturally explained by strong inhomogeneity of H2 distribution.

3. Summary